The audible frequency range (the frequency of periodic vibration audible to the human ear) is from about 50 Hz to about 22 kHz, but hearing degenerates with age and most adults find it difficult to hear above about 14-15 kHz. Most of the energy of human speech signals is generally limited to the range from 250 Hz to 3.4 kHz. Thus, traditional voice transmission systems were limited to this range of frequencies, often referred to as the “narrowband.” However, to allow for better sound quality, to make it easier for listeners to recognize voices, and to enable listeners to distinguish those speech elements that require forcing air through a narrow channel, known as “fricatives” (‘s’ and ‘f’ being examples), newer systems have extended this range to about 50 Hz to 7 kHz. This larger range of frequencies is often referred to as “wideband” (WB) or sometimes HD (High Definition)-Voice.
The frequencies higher than the WB range—from about the 7 kHz to about 15 kHz—are referred to herein as the Bandwidth Extension (BWE) region. The total range of sound frequencies from about 50 Hz to about 15 kHz is referred to as “superwideband” (SWB). In the BWE region, the human ear is not particularly sensitive to the phase of sound signals. It is, however, sensitive to the regularity of sound harmonics and to the presence and distribution of energy. Thus, processing BWE sound helps the speech sound more natural and also provides a sense of “presence.”